Alzheimer's patients often experience rapid weight loss and deterioration in nutritional status. This has led to the suggestion that their energy expenditure is elevated relative to their body size. In this 3 yr proposal, our Major Aim is to examine whether total daily energy expenditure (TDEE), derived from doubly labeled water, and its components (resting metabolic rate and physical activity) are higher in Alzheimer's patients compared to normal elderly. We hypothesize that energy expenditure of Alzheimer's patients will not be elevated after normalizing for differences in fat-free mass. Thus, a reduced intake relative to their energy requirement will be implicated in their weight loss. The Secondary Aim of this proposal is to use the data collected to develop and cross-validate new models to predict energy requirements as derived from biological markers of TDEE in Alzheimer's patients and normal elderly and compare them to current energy recommendations. Accurate information to define suitable levels of energy intake is required for the prevention and treatment of the extremes of fatness and leanness that arise in Alzheimer's patients and healthy elderly. Current recommendations for energy requirements are based on out-dated food intake data and have not considered the influence of body composition and physical activity as important determinants of energy needs. These limitations have led to the suggestion that prescription of energy requirements should be based on the measurement of energy expenditure. We propose that energy requirements will be most accurately predicted from a marker of physical activity and fat-free mass in Alzheimer's patients and normal elderly. In combination, these experiments will use stable isotope methodology and indirect calorimetry to examine energy metabolism in Alzheimer's patients and normal elderly, and offer new guidelines to predict energy requirements based on measures of energy expenditure.